1. Field of the Invention
This invention relates to a starting device for a discharge lamp. More particularly, it relates to a starting device which is so improved as to be suitable for the starting of a metallic vapor discharge lamp of comparatively high starting voltage, such as a high pressure mercury-vapor lamp, a metal halide-vapor lamp or a high pressure sodium-vapor lamp.
2. Description of the Prior Art
A high-pressure metallic vapor discharge lamp such as a mercury-vapor lamp, a metal halide-vapor lamp or a high pressure sodium-vapor lamp has a higher starting voltage than a low-pressure metallic vapor discharge lamp, such as a fluorescent lamp. Such contrivance is therefore required in order to start the high-pressure metallic vapor discharge lamp with a commercial AC power source (the effective voltage of which is as low as about 100-200 volts).
For example, a starting system as shown in FIG. 1 has been already proposed in order to start the mercury-vapor lamp by the use of a commercial 200-volt power source. In this system, a discharge tube 2 is connected to the commercial 200-volt power source 5 through a single choke coil ballast 1. An auxiliary electrode 8 is provided in proximity to one main electrode 7 of the discharge tube 2, and it is connected to the other main electrode 7' through a starting auxiliary resistance 9 (of about 20 k.OMEGA.).
In the case of employing the commercial 200-volt power source in this manner, the mercury-vapor lamp can be satisfactorily started even by the circuit which uses only the single choke coil ballast as illustrated in FIG. 1. In the case of employing a commercial 100-volt power source, however, effective starting is difficult to achieve with such a single choke coil system.
The metal halide-vapor lamp and the high pressure sodium-vapor lamp have a still higher starting voltage as compared to the mercury-vapor lamp described above. Therefore, they cannot be started by the foregoing single choke coil system even when the commercial 200-volt power source is used. In the case of these lamps, accordingly, special ballasts are employed which can generate voltages sufficiently higher than the available supply voltages at the time of starting. The ballasts are more complicated in construction and larger in size than the single choke coil ballast, and lead to economic disadvantages.
A starting system as shown in FIG. 2 has therefore been recently proposed for the high pressure sodium-vapor lamp. In this system, a discharge tube 2 is connected to a power source 5 through a choke coil ballast 1, and a bimetal switch 3 and a heating filament 4 are connected in series between electrodes 7 and 7' of the discharge tube 2. The bimetal switch 3 is kept closed at the normal temperature, and it is opened when heated above a certain temperature. In operation, when an AC voltage is applied from the power source 5, a current flows through a closed circuit consisting of the choke coil ballast 1, bimetal switch 3 and heating filament 4, and the heating filament 4 is red-heated. When the bimetal switch 3 is heated and opened by heat from the filament 4, a high voltage pulse P is generated in superposition on a supply voltage V.sub.s as shown in FIG. 3. Upon generation of this pulse voltage, the discharge tube 2 is fired. When the discharge tube 2 is started, the bimetal switch 3 is retained in the open state by heat which is developed by the discharge lamp itself. The height V.sub.p of the pulse voltage varies depending on the current phase at the time when the bimetal switch 3 is opened, and the highest pulse voltage V.sub.p max is produced in the case where the bimetal switch is opened at the maximum or peak value of the current. The highest pulse voltage V.sub.p max varies depending on the short-circuit current I.sub.s = V.sub.s /R.sub.H where R.sub.H denotes the resistance value of the heating filament 4 being red-heated by the flow of the current, and the value V.sub.p max is greater as the current I.sub.s is greater. By way of example, FIG. 4 shows the relationship between I.sub.s and V.sub.p max in the case of employing a choke coil for a mercury-vapor lamp of 400 watts.
However, the propability that the highest pulse voltage V.sub.p max will be actually attained (that is, the probability that the bimetal switch 3 will be opened when the flowing current is the maximum) is small. Usually, pulse voltages of 1/2 V.sub.p max or so are often obtained. For the starting of the lamp, a higher pulse voltage V.sub.p is more effective. Ordinarily, however, the short-circuit current I.sub.s is selected within a range of 1-2 amperes, so that when the supply voltage V.sub.s is 200 volts the resistance value R.sub.H of the heating filament at the high temperature is set within a range of 100-200 ohms. When the short-circuit current I.sub.s is greater than the specified values, the pulse voltage to be generated becomes too high, and it is feared that it will result in dielectric breakdown of the choke coil ballast. Conversely, when the short-circuit current I.sub.s is smaller than the specified values, the pulse voltage to be produced becomes too low, and the effect of aiding in the starting of the discharge lamp becomes insufficient.
As the heating filament 4, which serves to heat the bimetal switch 3 and which also serves to set the value of the short-circuit current I.sub.s within the appropriate range, a tungsten filament is usually used. The reason therefor is that, in the case where I.sub.s = 1-2 amperes and R.sub.H = 100-200 ohms as stated above, the heating filament 4 must withstand heat of 200-400 watts, so a fixed resistor is unreasonable in capacity. When the resistance R.sub.H of the tungsten filament at the high temperature is made 100-200 ohms, the resistance R.sub.o thereof at the normal temperature is at most about 2-3 ohms.
The starting system illustrated in FIG. 2 has the following disadvantages. Firstly, as previously stated, in the case where the bimetal switch is opened in the phase in which the short-circuit current is small, a sufficiently high pulse voltage is not produced, so that the discharge lamp is not always started by one operation of the bimetal switch. In particular, a discharge lamp, such as the metal halide-vapor lamp which exhibits a delay of several hundreds of milliseconds from the application of a voltage to the initiation of arc discharge, requires 5-6 switching operations before starting in many cases. Therefore, a long time is required for the starting. Secondly, in the case where the height V.sub.p of the pulse voltage generated at the opening of the bimetal switch is small and therefore the discharge lamp cannot be started, the bimetal switch is cooled and closed again, and at this time, the contact element of the switch gives rise to a chattering phenomenon. Pulse voltages generated by the chattering phenomenon are much higher than the voltage V.sub.p generated at the opening of the bimetal switch, and reach 20-30 kilovolts. In the case where the discharge lamp is normal, the pulse voltages generated by the chattering phenomenon of the switch cannot piece are absorbed by the action of the discharge lamp as a discharge gap and therefore do no harm. However, in the case where the starting voltage of the discharge lamp has become high at the last stage of the lifetime of the lamp or by any other cause, the discharge lamp no longer functions as a good discharge gap. In such case, accordingly, the high pulse voltages as discharged above cannot be absorbed, and it is possible that dielectric breakdown of the ballast winding will result.